The voltage derived from a photovoltaic solar panel d.c. source is a function of the sunlight incident on the panel and the temperature of the panel. Therefore, the voltage derived from the panel is highly variable due to the long and short term variations of solar energy incident on the panel. The available power per unit area of the solar panel goes from a nocturnal value of zero to a maximum value at the middle of a sunny day. The power derived from the panel may have a relatively constant value during the entire daytime hours on hazy days. The power derived from the panel can also have sudden variations due to clouds transiently in the field of view of the panel to the sun.
Solar panel systems are presently being designed wherein a load is driven directly by the panel output voltage without the intermediary of an energy storage device, such as a rechargeable battery. Such systems usually include, for efficient operation, a chopping d.c. to a.c. inverter, with associated control circuitry. One particularly advantageous chopper is disclosed in my copending, commonly assigned application, Ser. No. 846,696, filed Oct. 31, 1977, entitled "Synthesizer Circuit for Generating Three-Tier Wave Forms," now U.S. Pat. No. 4,135,235, wherein a pseudo-sinusoidal, notched, multilevel waveform is derived from a d.c. source and supplied to an a.c. load, such as a synchronous motor. This inverter, preferably controlled by a variable frequency function generator as disclosed in the commonly assigned application Ser. No. 890,460 of Baker et al., filed Mar. 27, 1978, entitled "Controller for Synthesizer Circuit for Generating Three-Tier Wave Forms," now U.S. Pat. No. 4,167,775, is such that the power output and current supplied from the d.c. source to the a.c. load increase proportionately with fundamental chopping frequency of the inverter. It is desired to supply maximum power from the panel to the inverter and to a load. However, the maximum power which can be derived from the panel is variable, as indicated above.
For any particular incident solar energy level at a given temperature, the output power of the panel is a function of the panel load, which in turn controls the panel output voltage and current. The output of a photovoltaic solar panel can thus be represented as a family of curves for the panel voltage versus current output parameters. The open circuit voltage and short circuit current of each curve in the family both increase the level of incident solar energy on a panel increases for a given temperature. Each of the curves has a maximum power point, at which the curve has a slope of minus one. Each of the curves has slopes with absolute values greater than one and less than one for currents respectively greater than and less than the maximum power point.
Equipment to track the maximum available power from the panel and to control the chopping frequency of the inverter in response thereto must be shut down in an orderly fashion when the available power from the solar panel is reduced below some minimum value necessary to maintain control of the inverter. Also, in day-night and night-day transitions, it is necessary to remove power from the inverter and control circuitry and supply power to the control circuitry and inverter in an orderly manner, while always insuring that sufficient control power exists to meet stability requirements of the inverter and the load.
It is, accordingly, an object of the present invention to provide a new and improved circuit and system for providing orderly control of the power applied by a photovoltaic solar panel d.c. source to a load despite varying long and short term solar conditions of the panel.
Another object of the invention is to provide a system and circuit for controllig the power applied by a photovoltaic solar panel d.c. source to an inverter having a chopping frequency controlled so that maximum available power from the panel is supplied to the inverter and orderly control for the turn-on and turn-off of the inverter and control circuitry therefore is provided.
An additional object of the invention is to provide a new and improved system and circuit for selectively supplying power from a photovoltaic solar panel d.c. source to a main load and control circuitry for the load.